Fighters of the Islamic State of Iraq and Syria stand guard at a checkpoint in the northern Iraq city of Mosul on June 11. Since Tuesday, black clad ISIS fighters have seized Iraq’s second biggest city, Mosul, and Tikrit, the hometown of former dictator Saddam Hussein, as well as other towns and cities north of Baghdad. REUTERS/Stringer

Islamist insurgents may have seized millions of dollars in cash as they looted banks while seizing the Iraqi city of Mosul this week, a windfall that the al-Qaeda-inspired group could use to expand territory and fund further attacks, U.S. officials said.

The group, known as the Islamic State of Iraq and Syria, or ISIS, “appears to have overrun several banks,” a U.S. official said. The official said that it was unclear how much cash the branches kept in their vaults but that banks across Iraq’s second largest city had “sizeable assets.”

Experts said the total could reach into tens of millions of dollars, but expressed skepticism of reports that ISIS may have seized as much as $400 million.

But even a percentage of that sum would significantly bolster the capabilities of a violent Islamist organization that has routed Iraqi forces while moving south through cities toward the nation’s capital in recent days.

“What it does is it fuels their momentum,” said Juan Zarate, a former senior U.S. Treasury Department official who was also Deputy National Security Adviser to President George W. Bush. “It gives them the ability to pay fighters, pay families, buy weapons and create alliances – all the things that money facilitates.”

The cash is in addition to weapons, U.S.-provided military vehicles and other resources and equipment that ISIS seized as part of its move across Mosul, a city of about 2 million people north of Baghdad. The group grew out of an al-Qaeda affiliate in Iraq that was largely shattered by the end of the U.S. involvement in the war there, but has since regrouped and now controls territory that straddles the country’s border with Syria.

News reports citing Iraqi regional governor Atheel al-Nujaifi, whose province includes Mosul, indicated that ISIS may have made off with as much as $500 billion Iraqi dinars, more than $400 million. U.S. officials and experts questioned whether branches would keep such amounts on hand, especially as violence has spiked in the country.

Still, a former U.S. intelligence official noted that Iraq and other countries in the Middle East are “still cash and carry. It wouldn’t be terribly surprising to me if they were sitting on large amounts of cash – amounts that we would consider startling.”

ISIS was already thought to have a steady stream of funding from criminal enterprises including kidnappings and smuggling as well as donations from wealthy Islamists. Zarate described the group’s march across northern Iraq as “an all-out disaster. They have men, money, materiel and momentum. This is a crisis moment for the region, our allies and our country.”